Technical Field
Embodiments of the invention relate generally to power plant technology and, more specifically, to a system and method for heating one or more components of a heat recovery steam generator.
Discussion of Art
Heat recovery steam generators (“HRSGs”) are devices that recover thermal energy from heat-containing mediums such as flue gas produced via combustion of a fuel, and/or any other liquid, gas, and/or plasma that contains recoverable thermal-energy. Many HRSGs recover heat from a heat-containing medium by exposing one or more components to the heat-containing medium. In such HRSGs, the evaporator generates steam by absorbing/recovering some of the thermal energy from the heat-containing medium. The super heater then heats the generated steam by absorbing/recovering more of the thermal energy from the heat-containing medium. The heated steam is then typically distributed to a steam consuming device and/or process, e.g., a steam powered turbine for generating electrical power. A similar process applies to supercritical heat recovery devices noting the distinction in working fluid properties.
Many power plants, known as combined cycle power generation plants, often use HRSGs to recover thermal energy from a flue gas produced by a primary generator such as a gas-powered turbine. The recovered thermal energy is then used to power a secondary generator.
In such power plants, however, it is possible that pressurize parts of the HRSG, and in particular the evaporator, steam drum, and the super heater, experience thermal stresses during plant start up due to temperature differences between the pressurize parts and the temperatures and pressures of the produced steam. Such thermal stresses can potentially degrade the service lives of the pressurize parts—especially pressurize parts having thick walls, dissimilar metal welds, austenitic grade metallurgy, and/or any combination of these therein. As combined cycle efficiencies continue to increase, the presence of thick walled components, dissimilar metal welds and austenitic metallurgy becomes increasingly requisite.
Notwithstanding these mechanical design considerations, e.g., thick wall components, dissimilar metal welds, and/or austenitic grade metallurgy, it is often necessary to minimize the combined cycle start up time. Pressure parts with thick walls, dissimilar metal welds, and/or austenitic grade metallurgy, however, require a longer start up time to mitigate damage that could result from thermal stresses. As can be appreciated, faster start up times are at odds with equipment lifetime. Thus, many power plants preheat pressurize parts.
Electrical heat tracing and auxiliary steam preheating are two common preheating practices. For electrical heat tracing, a heating wire element is wrapped around a pressure part component, and in particular, component areas at risk of significant thermal stress borne from the start up gradients mentioned above. Components are preheated from the outside inward to reduce the through wall stresses that occur during the start up events mentioned. One principal benefit of electrical heat tracing is the heat input is well quantified and a design can be made to cater to the given start up sequence. In practice, however, this design approach is cost prohibitive due to hardware and operation reasons. Namely, the heating element is very costly—particularly if large quantities are required. Secondly, the power consumption is similarly non-trivial due to the amount of heat input required.
With an auxiliary steam source, steam is admitted from an external source to the subject pressure part; hence heat is provided from the inside outward to reduce the through wall stresses that occur during the previously mentioned start up events. For instance, high pressure evaporators commonly utilize an auxiliary steam source to keep the pressure part warm which allows for faster steam production. A drawback of such a device is the capital equipment of this additional item as well as the operating costs, e.g., fuel and maintenance cost to run the auxiliary boiler.
In view of the above, what is needed is an improved system and method for preheating one or more components of an HRSG.